Optical magnetic field sensor probe having drum lenses

ABSTRACT

An optical magnetic field sensor probe for detecting a magnetic field to be measured as an output light intensity, by disposing a polarizer, a magneto-optical device, and an analyzer mutually different in the transmission and polarization direction with respect to said polarizer, along the running direction of light, has an input optical fiber for feeding into to said polarizer through a first lens, and an output optical fiber for emitting an output light from said analyzer through a second lens, wherein said input optical fiber, said first lens, said magneto-optical device, said second lens and said output optical fiber are composed in a confocal optical system, and said first lens and said second lens are drum lenses.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magneto-optical device using theFaraday effect, and to an optical magnetic field sensor probe fordetecting a magnetic field by using the same to measure the intensity ofthe magnetic field.

2. Related Art of the Invention

As a method of measuring the magnetic field intensity generated aroundan electric current by using light, an optical magnetic field sensorcombining a magneto-optical device having the Faraday effect and opticalfiber is known. Such an optical magnetic field sensor provides highinsulation and is free from the effects of electromagnetic inductionnoise, and owing to such advantages, already, it is realized as a sensorfor detecting accidents of high voltage distribution lines in theelectric power field (Journal of the Institute of Electrical Engineersof Japan, Section B, Vol. 115, No. 12, p. 1447, 1995). Recently,moreover, there is a mounting need for higher performance for thisinstrument, and an optical magnetic field sensor of high precision andsmall size is demanded.

As the optical magnetic sensor making use of the Faraday effect,hitherto, the sensor heads as shown in FIGS. 16(a) and 16(b) have beendisclosed (see Journal of Japan Society of Applied Magnetics, Vol. 19,No. 2, p. 209, 1995, and IEEE Transactions on Magnetics, Vol. 31, p.3191, 1995). In FIGS. 16(a) and 16(b), magneto-optical devices 1 of rareearth iron garnet material are disposed in a magnetic field H. Thesensor head in FIG. 16(a) constitutes a collimated optical system usingcollimated lenses 24a, 24b. Herein, the rare earth iron garnet materialused as the magneto-optical device 1 measures 3 mm square and 60 μm infilm thickness. Optical fibers 6a and 6b are multi-mode optical fiberswith a core diameter of 200 μm. In a polarizer 2 and an analyzer 3a,polarizing beam splitters of a 5 mm cube are used, and the polarizer 2and analyzer 3a are disposed so that the direction of polarization maybe mutually different by 45 degrees. The light entering from an inputoptical fiber 6a is transformed into a parallel light beam by thecollimated lens 24a. It is further transformed into a straight polarizedlight beam by the polarizer 2, and passes through the magneto-opticaldevice 1, and the plane of polarization is rotated in proportion to theintensity of the magnetic field by the Faraday effect. The rotatedstraight polarized light passes through the analyzer 3a different by 45degrees in the transmission and polarization direction with respect tothe polarizer 2, and is reflected by a total reflection mirror 4,condensed by the collimated lens 24b, and is focused on the outputoptical fiber 6b. In such an optical system, the analyzer 3a is fixed,the light output from the polarizer 3a is utilized in one port only, andhence it is called the non-differential fixed analyzer method, in whichthe change in the magnetic field intensity is converted into a change inquantity of light so as to be measurable. In the optical magnetic fieldsensor shown in FIG. 16(a), of the light diffracted by themultiple-domain structure of rare earth iron garnet material serving asthe magneto-optical device 1, only the 0th-order light is received, andtherefore it is hitherto unveiled that the increases as the magneticfield becomes higher.

On the other hand, the sensor head in FIG. 16(b) constitutes a confocaloptical system using spherical lenses 25a, 25b as the lenses, andforming a beam waist at the position of the magneto-optical device 1.Thus, the light diffracted by the rare earth iron garnet material can bereceived up to a high order, so that the linearity is improved. In FIG.16(b), in order to shorten the optical path length so as to form a beamwaist at the position of the magneto-optical device 1, a 3 mm squareglass polarizing plate is used in the analyzer 3b. The spherical lenses25a, 25b, are 3 mm in diameter, being made of material BK-7 with arefractive index of 1.517, and the sensor head measures 12 mm in widthand 20 mm in length. These optical magnetic field sensors are installedin the gap of an iron core 16 as shown in FIG. 9 (a block diagram of anoptical transformer using the optical magnetic field sensor probe of theinvention), and used as optical transformers. Therefore, the smaller thewidth of the sensor head, the narrower the gap that may be formed, sothat an optical transformer of high sensitivity may be realized.

As the magneto-optical device 1 used in such a sensor, the rare earthiron garnet material as shown in formula 2 is disclosed (see TechnicalResearch Report of Electronics, Information and Communication Society ofJapan, OQE92-105, 1992). In this prior art, by replacing Y with Bi orGd, a magneto-optical device of excellent temperature characteristic isrealized. The chemical formula of the crystal used in this prior art isshown in formula 2.

    Bi.sub.1.3 Gd.sub.0.1 La.sub.0.1 Y.sub.1.5 Fe.sub.4.4 Ga.sub.0.6 O.sub.12(Formula 2)

The linearity and temperature characteristic of the optical magneticfield sensor shown in FIG. 16(b) fabricated by using thismagneto-optical device are shown in FIG. 17 and FIG. 18. As shown inFIG. 17, a favorable linearity of 1.0% or less is realized in a magneticfield range of about 25 Oe to 300 Oe. However, to measure a weakmagnetic field of less than 25 Oe, the linearity error is large, and apractical problem is noted. The measuring range is narrow, only up to300 Oe, and an optical magnetic field sensor having a wider measuringrange is desired. FIG. 18 shows the result of measuring changes ofsensitivity depending on temperature by using two kinds of sensoroptical systems, that is, the collimated optical system shown in FIG.16(a) and the confocal optical system shown in FIG. 16(b), by using themagneto-optical device shown in formula 2. The change rate ofsensitivity is normalized by room temperature, and the applied magneticfield is an alternating-current magnetic field of 50 Oe and 60 Hz. Inthe optical magnetic field sensor shown in FIG. 16(a) composed of thecollimated optical system for receiving 0th-order diffracted light onlyas indicated by bullet marks in FIG. 18, the temperature dependentchange of sensitivity of 1.0% or less is obtained. However, in the caseof using the magneto-optical device shown in formula 2 in the opticalmagnetic field sensor shown in FIG. 16(b), a positive characteristic ofabout 10% of temperature dependent sensitivity change rate is shown asindicated by blank circle marks in FIG. 18. That is, the opticalmagnetic field sensor in FIG. 16(b) is excellent in linearity, but has aserious problem in the temperature characteristic of the sensitivity.

Therefore, in the prior art, an optical magnetic field sensor satisfyingthe contradictory problems of favorable linearity and favorabletemperature characteristic cannot be realized. Accordingly, an opticalmagnetic field sensor of smaller size and higher precision is demanded.

SUMMARY OF THE INVENTION

The invention is devised to solve such problems of the conventionaloptical magnetic field sensor, and it is an object thereof to present anoptical magnetic field sensor high in measuring precision, small insize, and easy to assemble, while satisfying the requirements ofexcellent linearity and temperature characteristic, and wide measuringrange.

To solve the above problems, the invention constitutes an opticalmagnetic field sensor probe of very small size, that improves thelinearity in a wide measuring range, and further improves thetemperature characteristic by employing a specific material composition.

To realize a sensor head of small size, drum lenses are formed by usingspherical lenses of high refractive index and polishing of theirperipheral area, and two drum lenses are fixed to a holder to fabricatea novel small-sized drum lens holder. Using such a drum lens holder, anoptical magnetic field sensor of 4 mm in width is constituted.

Moreover, as the magneto-optical device to be used in this opticalmagnetic field sensor probe, a rare earth iron garnet material ispresented, which is expressed in the following formula 1, where thevalue of x is defined in a range of 0.84≦x≦1.10, the value of y in arange of 0.73≦y≦1.22, the value of z in a range of 0.02≦z≦0.03, and thevalue of w in a range of 0.27≦w≦0.32.

    (Bi.sub.x Gd.sub.y R.sub.x Y.sub.3-x-y-z)(Fe.sub.5-w Ga.sub.w)O.sub.12(Formula 1)

where R is at least one element selected from the rare earth elements.

Further, to improve the linearity of a low magnetic field, taking noteof the magnitude of the magnetic field coercive force (Hw) of the rareearth iron garnet material, a rare earth iron garnet material of smallmagnetic wall coercive force is manufactured in order to decrease thelinearity error. For this purpose, herein, the dependence of themagnetic wall coercive force on the film thickness and annealing effecthave been discovered. According to the present invention, the magneticwall coercive force can be decreased by properly regulating the filmthickness in a certain composition of rare earth iron garnet material.Also by heat treatment of the rare earth iron garnet material expressedin formula 1 at a high temperature, the magnetic wall coercive force canbe decreased. The rare earth iron garnet material thus optimized in filmthickness and heat treatment of the film is applied in the opticalmagnetic field sensor probe as the magneto-optical device.

The action and effect of the invention having such constitution aredescribed below. By the constitution of the invention, the width of theoptical magnetic field sensor is extremely reduced, and the number ofparts is smaller, so that its assembly is easier. In particular, by theuse of the drum lens holder, the reliability is also enhanced. Accordingto the optical magnetic field sensor probe of the invention, the qapinterval of the void iron core can be decreased. Therefore, an opticaltransformer of high sensitivity to input current is realized. At thesame time, the head of the optical transformer can be also formed with avery small size.

Furthermore, the magneto-optical device of the invention is used in theoptical system for receiving diffracted light up to a high order by amultiple domain structure of the magneto-optical device. That is, byconstituting the optical magnetic field sensor probe by using themagneto-optical device of the invention, diffracted light of high ordercan be received, so that a sensor of excellent linearity can berealized. Moreover, since the saturation magnetic field of themagneto-optical device is designed to be larger than in the conventionalmagneto-optical device, it is possible to measure with excellentlinearity up to a high magnetic field. A superior temperaturecharacteristic is further realized by using a specific composition ofrare earth iron garnet material for the optical magnetic field sensorprobe of the invention. Thus, the optical magnetic field sensor probe ofsmall size and high precision is realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an embodiment of an optical magnetic fieldsensor probe according to the invention.

FIG. 2 is a diagram showing another embodiment of an optical magneticfield sensor probe according to the invention.

FIG. 3A is an outline diagram of a drum lens holder according to theinvention.

FIGS. 3B(a)-3B(e) are diagrams showing the steps of the assembly processof the optical magnetic field sensor probe for a second embodiment ofthe invention.

FIG. 4 is a diagram showing yet another embodiment of an opticalmagnetic field sensor probe according to the invention.

FIG. 5 is a diagram showing another embodiment of an optical magneticfield sensor probe according to the invention.

FIG. 6 is a diagram showing yet another embodiment of an opticalmagnetic field sensor probe according to the invention.

FIG. 7 is a diagram showing still another embodiment of an opticalmagnetic field sensor probe according to the invention.

FIG. 8 is a diagram showing the linearity of an optical magnetic fieldsensor probe according to the invention.

FIG. 9 is a block diagram of an optical transformer using an opticalmagnetic field sensor probe according to the invention.

FIG. 10 is a diagram showing the linearity of an optical transformerusing an optical magnetic field sensor probe according to the invention.

FIG. 11 is a diagram showing the sensitivity of an optical transformerusing an optical magnetic field sensor probe according to the invention.

FIG. 12 is a diagram showing the temperature characteristic of anoptical magnetic field sensor probe according to the invention.

FIG. 13 is a diagram showing the dependence of linearity error on themagnetic wall coercive force of an optical magnetic field sensor probeaccording to the invention.

FIG. 14(a) is a diagram showing a frequency spectrum at 3.4 Oe of anoptical magnetic field sensor according to the invention.

FIG. 14(b) is a diagram showing a frequency spectrum at 105 Oe of anoptical magnetic field sensor according to the invention.

FIG. 15(a) is a side view showing a heat treatment method of amagneto-optical device according to the invention.

FIG. 15(b) is a perspective view showing a heat treatment method of amagneto-optical device according to the invention.

FIG. 16(a) is a diagram showing an internal structure of an opticalmagnetic field sensor using a conventional collimated optical system.

FIG. 16(b) is a diagram showing an internal structure of an opticalmagnetic field sensor using a conventional confocal optical system.

FIG. 17 is a diagram showing the linearity of a conventional opticalmagnetic field sensor.

FIG. 18 is a diagram showing the temperature characteristic of aconventional optical magnetic field sensor.

REFERENCE NUMERALS

1 Magneto-optical device

2 Polarizer

3 Analyzer

4 Total reflection mirror

4a Evaporated total reflection mirror

5a, 10a Input side drum lenses

5b, 10b Output side drum lenses

6a Input optical fiber

6b Output optical fiber

7 Sensor case

8a, 8b Input and output optical fiber cords

9a Input light

9b Output light

11 Drum lens holder

12 Ferrule

13a, 13b Bent hemispherical-ended optical fibers

14a, 14b Bent optical fibers for holding plane of polarization

15a, 15b Plastic optical fibers

16 Void iron core

17 Optical magnetic field sensor probe

18 Signal processing circuit

19 Conductor

20 Substrate

21 Rare earth iron garnet material (grown film)

22 SGGG crystal substrate

23 Ceramic tray

24a, 24b Collimated lenses

25a, 25b Spherical lenses

PREFERRED EMBODIMENTS

Referring now to the drawings, embodiments of the invention aredescribed in detail below.

A first embodiment of the invention relates to an optical magnetic fieldsensor probe for detecting a magnetic field to be measured as an outputlight intensity, by disposing a polarizer, a magneto-optical device, andan analyzer mutually different in the transmission and polarizationdirection with respect to the polarizer, along the running direction ofa light path. The embodiment also comprises an input optical fiber forfeeding light into to the polarizer through a first lens, and an outputoptical fiber for emitting an output light from the analyzer through asecond lens, wherein the input optical fiber, the first lens, themagneto-optical device, the second lens and the output optical fiber arecomposed in a confocal optical system, and the first lens and the secondlens are drum lenses.

A second embodiment of the invention relates to an optical magneticfield sensor probe for detecting a magnetic field to be measured as anoutput light intensity, by disposing a polarizer, a magneto-opticaldevice, and an analyzer mutually different in the transmission andpolarization direction with respect to the polarizer, along the runningdirection of a light path. The second embodiment also comprises an inputoptical fiber for feeding light into to the polarizer through a firstlens, and an output optical fiber for emitting an output light from theanalyzer through a second lens, wherein the input optical fiber, thefirst lens, the magneto-optical device, the second lens and the outputoptical fiber are composed in a confocal optical system, and a lensholder is disposed for incorporating the first lens and the second lens.

A third embodiment of the invention relates to an optical magnetic fieldsensor probe, wherein the first lens and the second lens are lenseshaving a refractive index of 1.517 or more.

A fourth embodiment of the invention relates to an optical magneticfield sensor probe for detecting a magnetic field to be measured as anoutput light intensity, by disposing a polarizer, a magneto-opticaldevice, and an analyzer mutually different in the transmission andpolarization direction with respect to the polarizer, along the runningdirection of a light path. The fourth embodiment also comprises an inputoptical fiber for feeding light into the polarizer through a first lensand a first mirror, and an output optical fiber for emitting an outputlight from the analyzer through a second lens and a second mirror,wherein the input optical fiber, the first lens, the first mirror, themagneto-optical device, the analyzer, the second lens, and the outputoptical fiber are composed in a "]" (which is ] of Japanesekatakana)-shaped optical system, and the first mirror and the secondmirror are formed by being directly evaporated to a case incorporatingthe optical system.

A fifth embodiment of the invention relates to an optical magnetic fieldsensor probe for detecting a magnetic field to be measured as an outputlight intensity, by disposing a polarizer, a magneto-optical device, andan analyzer mutually different in the transmission and polarizationdirection with respect to the polarizer, along the running direction ofa light path, wherein an input optical fiber provided at one end of themagneto-optical device across from the polarizer, and an output opticalfiber provided at the other end of the magneto-optical device acrossfrom the analyzer are optical fibers having a lens directly at the frontend.

A sixth embodiment of the invention relates to an optical magnetic fieldsensor probe, wherein the optical fibers are optical fibers bent at thefront end.

A seventh embodiment of the invention relates to an optical magneticfield sensor probe for detecting a magnetic field to be measured as anoutput light intensity, by disposing an input optical fiber having afunction for holding a plane of polarization, a magneto-optical device,and an output optical fiber having a function of holding a plane ofpolarization different from the direction of polarization of the inputoptical fiber, along the running direction of the light path.

An eighth embodiment of the invention relates to an optical magneticfield sensor probe, wherein the optical fibers for holding the planes ofpolarization are bent at the front end.

A ninth embodiment of the invention relates to an optical magnetic fieldsensor probe for detecting a magnetic field to be measured as an outputlight intensity, by disposing an input optical fiber, a polarizer, amagneto-optical device, an analyzer mutually different in thetransmission and polarization direction with respect to the polarizer,and an output optical fiber, along the running direction of the lightpath, wherein the input optical fiber and the output optical fiber areall-plastic optical fibers.

A tenth embodiment of the invention relates to an optical magnetic fieldsensor probe, wherein the all-plastic optical fibers are bent at thefront end.

An eleventh embodiment of the invention relates to a magneto-opticaldevice using a rare earth iron garnet material, wherein the rare earthiron garnet material is expressed in the following formula, where thevalue of x is defined in a range of 0.84≦x≦1.10, the value of y in arange of 0.73≦y≦1.22, the value of z in a range of 0.02≦z≦0.03, and thevalue of w in a range of 0.27≦w≦0.32.

    (Bi.sub.x Gd.sub.y R.sub.z Y.sub.3-x-y-z)(Fe.sub.5-w Ga.sub.w)O.sub.12(Formula 1)

where R is at least one element selected from the rare earth elements.

A twelfth embodiment of the invention relates to a magneto-opticaldevice, wherein the rare earth iron garnet material is formed byepitaxial growth on a garnet crystal substrate.

A thirteenth embodiment of the invention relates to a magneto-opticaldevice, wherein the garnet crystal substrate is a Ca--Mg--Zr substituenttype Gd₃ Ga₅ O₁₂ substrate.

A fourteenth embodiment of the invention relates to a magneto-opticaldevice, wherein the rare earth iron garnet material is heat-treated.

A fifteenth embodiment of the invention relates to a magneto-opticaldevice, wherein the magnetic wall coercive force of the rare earth irongarnet material is 0.2 Oe or less.

The invention is further described below by referring to the embodimentsin detail.

(Embodiment 1)

FIG. 1 is a diagram showing an optical magnetic field sensor probeaccording to a first embodiment of the invention. A magneto-opticaldevice 1 in FIG. 1 is made of a rare earth iron garnet materialexpressed in formula 1. The broken line in FIG. 1 shows the traces ofrays of light, which indicate a confocal optical system. To reduce thewidth of the sensor head, drum lenses 5a and 5b of 3 mm in lens radiusand 2 mm in drum diameter, being made of material BK-7 (refractive index1.517), are used. A polarizer 2 and an analyzer 3 are 2 mm square glasspolarizing plates, and total reflection mirrors 4 are 2 mm squaredielectric multi-layer film evaporation mirrors. Optical fibers 6a and6b are multi-mode optical fibers with a core diameter of 200 μm. Asshown in FIG. 1, by fixing the constituent components in a case 7, anoptical magnetic field sensor probe of a small size of 6 mm in width,half of the conventional size, is realized.

A beam 9a emitted from a light source of a signal processing circuit 18in FIG. 9 is guided into the sensor head through an optical fiber cord8a, and enters from the optical fiber 6a. The incident light iscondensed by the drum lens 5a, and the optical path is bent to 90degrees by the total reflection mirror 4 to be formed into a straightpolarized light by the polarizer 2, and it passes through themagneto-optical device 1. At this time, the incident light 9a forms abeam waist at the position of the magneto-optical device 1. When amagnetic field is applied vertically to the film surface of themagneto-optical device 1, the plane of polarization of the light isrotated by Faraday's rotation. Since the axes of polarization of thepolarizer 2 and analyzer 3 are angularly configured by a mutual rotationof 45 degrees, the magnetic field intensity is converted into a lightintensity. The light passing through the analyzer 3 is bent again in theoptical path to 90 degrees by the total reflection mirror 4, and iscondensed by a drum lens 5b, and is focused on an optical fiber 6b.

In this optical magnetic field sensor probe, the optical configurationof the input optical fiber 6a, the magneto-optical device 1, and of themagneto-optical device 1 and output optical fiber 6b is a confocaloptical system through the lenses, and is composed almost linearlysymmetrically on both sides of the magneto-optical device 1. In thisway, by composing a confocal optical system linearly symmetrical to themagneto-optical device 1, the diffracted light of the magneto-opticaldevice having magnetic domains can be received up to a high-order beam,and a sensor of excellent linearity is realized.

(Embodiment 2)

FIG. 2 is a diagram showing an optical magnetic field sensor probeaccording to a second embodiment of the invention. The optical magneticfield sensor probe in FIG. 2 differs from the sensor in FIG. 1 in thecondenser lenses, which are drum lenses 5a and 5b of material SF-8(refractive index 1.689), lens radius of 2 mm, and drum diameter of 1.25mm. Further differently, input side drum lens 5a and output side drumlens 5b are incorporated in a ceramic or non-magnetic stainless steelholder 11. The drum lens incorporated holder 11 is shown in FIG. 3A. Inthe parallel penetration holes of the holder, the two drum lenses 5a, 5bare fixed.

FIGS. 3B(a)-3B(e) are diagrams showing the assembly process of theoptical magnetic field sensor probe according to the second embodimentof the invention. The drum lens holder 11 has two drum lenses 10 insideguide holes (FIG. 3B). When assembling, after adhering the optical fiberto a ferrule 12, the end surface of the ferrule is polished precisely.The optical fiber cord 8 fixed to the ferrule 12 is inserted into thedrum lens holder 11 shown in FIG. 3B(a), and is adhered and fixed (FIG.3B(b)). On the other hand, by making use of the bottom and wall of theinside of the case 7, the magneto-optical element 1, polarizer 2,analyzer 3, and total reflection mirror 4 are fixed preliminarily (FIG.3B(c)). Finally, the drum lens holder 11 fixing the optical fiber cord 8is fixed at a specified position of the case 7 (FIG.3B(d)), and theoptical magnetic field sensor probe is completed (FIG. 3B(e)). In thisway, by using the drum lens holder 1 which was not used conventionally,adjustment of the optical axes is easy and reliability is much improved.As compared with embodiment 1, the size is much smaller. The opticalmagnetic field sensor probe shown in FIG. 2 measures 4 mm in width and20 mm in length.

Moreover, when using a drum lens of material TaF-3 having a higherrefractive index (1.804) in a confocal optical system, the distancebetween the input side lens 5a and output side lens 5b can be shortened,and the probe width is further reduced.

(Embodiment 3)

FIG. 4 is a diagram showing an optical magnetic field sensor probeaccording to a third embodiment of the invention. The optical magneticfield sensor probe in FIG. 4 differs from the sensor in FIG. 2 in theformation of the mirrors by directly evaporating the dielectricmulti-layer film to the inside of the case 7 in order to furtherdecrease the number of parts. In this constitution, the step foradhering the dielectric multi-layer film evaporation mirrors 4 to thecase 7 is omitted, and the assembling in embodiment 2 is furthersimplified, and the practical reliability is further enhanced.

(Embodiment 4)

FIG. 5 is a diagram showing an optical magnetic field sensor probeaccording to a fourth embodiment of the invention. The optical magneticfield sensor probe in FIG. 5 differs from the optical magnetic fieldsensor in FIG. 1 in the omission of the lenses in order to decreasefurther the number of parts. Instead, bent hemispherical-ended opticalfibers 13a, 13b, bent at the end to 90 degrees, are used as the inputoptical fiber and the output optical fiber. By using the benthemispherical-ended optical fibers 13, too, the sensor optical system iscomposed in a confocal optical system the same as in embodiment 1 andembodiment 2. By this arrangement, a 6 mm wide small optical magneticsensor probe of high precision can be manufactured.

(Embodiment 5)

FIG. 6 is a diagram showing an optical magnetic field sensor probeaccording to a fifth embodiment of the invention. The optical magneticfield sensor probe in FIG. 6 differs from the sensor in FIG. 1 in theomission of the lenses, polarizer and analyzer in order to decrease thenumber of parts further and to decrease the width of the head, and inthat the front end bent optical fibers 14a and 14b having a function ofholding the plane of polarization are used as the input optical fiber14a and the output optical fiber 14b. The planes of polarization of theinput optical fiber 14a and the output optical fiber 14b are disposed bymutually rotating them by 45 degrees. The width of the thus fabricatedoptical magnetic field sensor probe is only 4 mm, and the size istherefore very small.

In this embodiment, the same performance can be exhibited if the opticalfibers are optical fibers having a function for holding the plane ofpolarization by adding a polarizing plate to the front end of the bentoptical fibers, instead of using the optical fibers alone for holdingthe plane of polarization.

(Embodiment 6)

FIG. 7 is a diagram showing an optical magnetic field sensor probeaccording to a sixth embodiment of the invention. The optical magneticfield sensor probe in FIG. 7 differs from the sensor in FIG. 1 in theuse of plastic optical fiber (POF) 15, bent at the front end, as theoptical fiber in order to enhance the SNR characteristic and linearity.Generally, the POF is very large in core diameter, being about 1 mm, andthe quantity of light received in the photodetector is large. Besides,with a large core diameter, the diffracted light generated when usingrare earth iron garnet material in the magneto-optical device 1 can bereceived up to a high order. Thus, by using the POF, the quantity ofreceived light in the optical magnetic field sensor probe is large, sothat the SNR characteristic of the sensor is further enhanced.

In embodiment 5 and embodiment 6, in order to shorten the distancebetween the input optical fiber and output optical fiber, a glasspolarizing plate of a thin film thickness or a rare earth iron garnetfilm is used, which is reduced in film thickness by omitting the SGGGsubstrate. Thus, the input light diffracted by the multiple domainstructure of the rare earth iron garnet material can be received up to ahigh order by the output optical fiber, so that an optical magneticsensor probe of further excellent linearity can be composed.

Results of the linearity of the optical magnetic field sensor probes ofembodiment 1 to embodiment 6 are shown in FIG. 8. As clear from FIG. 8,the optical magnetic field sensor probes of the embodiments presentexcellent linearity in a wider magnetic field range by 200 Oe than inthe prior art shown in FIG. 17, and the linearity error is only 1.0% orless in a magnetic field range of up to 500 Oe.

Next, these optical magnetic field sensor probes were put in the gaps ofvoid iron cores, and optical transformers were thus composed. Thestructure is shown in FIG. 9. Herein, reference numeral 16 is an ironcore, 17 is an optical magnetic field sensor probe, 18 is a signalprocessing circuit, and 19 is a conductor. When used as the opticaltransformer, the results of measurement of linearity and sensitivity areshown respectively in FIG. 10 and FIG. 11. Measured data of conventionaloptical transformer is also provided, and it is known that bothlinearity and sensitivity are much improved by reducing the size. InFIG. 9, the optical transformer is composed by using the iron core 16,but by installing the optical magnetic field sensor probe directly inthe conductor without using the iron core, it was also possible tomeasure sufficiently.

(Embodiment 7)

An embodiment of a magneto-optical device of the invention is describedbelow by referring to the diagrams and a table. Table 1 is a compositiontable showing a seventh embodiment of the invention. Using a Bi₂ O₃--PbO--B₂ O₅ flux, a rare earth iron garnet material expressed informula 1 was formed on a Ca--Mg--Zr substituted Gd₃ Ga₅ O₁₂ substrateby crystal growth by the LPE method. The numerical value shows the ratioin composition of each element. To evaluate the temperaturecharacteristic of the magneto-optical devices shown in Table 1, themagneto-optical devices were used in the optical magnetic field sensorprobe shown in FIG. 2, and changes of sensitivity depending ontemperature were measured in a magnetic field range below the saturationmagnetic field of the magneto-optical device. The change rate of sensorsensitivity is shown in Table 1. The frequency of thealternating-current magnetic field is 60 Hz. The samples with # markedin the temperature characteristic in Table 1 are those outside the scopeof the invention. As for the asterisked samples in Table 1, the measureddata of changes of sensitivity depending on temperature are shown inFIG. 12.

                  TABLE 1                                                         ______________________________________                                        Composition of rare earth iron garnet                                                                   Change rate                                         material                  of sensor                                                                    Y                sensitivity                         Sample                                                                              Bi     Gd     La   3     Fe    Ga   (-20 to                             No.   x      y      z                                                         x-y-z 5-w    w      +80° C.)                                           ______________________________________                                        *1    1.27   0.12   0.12 1.49  4.41  0.59 #10.0%                              2     1.20   0.45   0.08 1.27  4.55  0.45 #8.0%                               3     1.15   0.55   0.05 1.25  4.65  0.35 #6.0%                               *4    1.13   0.64   0.03 1.20  4.73  0.27 #5.0%                               *5    1.10   0.73   0.03 1.14  4.73  0.27 3.0%                                *6    1.05   0.81   0.03 1.11  4.73  0.27 2.0%                                *7    0.98   0.92   0.03 1.07  4.72  0.28 1.0%                                *8    0.92   1.00   0.02 1.06  4.71  0.29 1.2%                                9     0.85   1.10   0.02 1.03  4.71  0.29 2.0%                                *10   0.84   1.22   0.02 0.92  4.68  0.32 3.0%                                *11   0.80   1.31   0.02 0.87  4.65  0.35 #5.0%                               ______________________________________                                    

The magneto-optical device of the invention is expressed in formula 1,and when the crystal composition is specified by the value of x definedin a range of 0.84≦x≦1.10, the value of y in a range of 0.73≦y≦1.22, thevalue of z in a range of 0.02≦z≦0.03, and the value of w in a range of0.27≦w≦0.32, in the temperature range of -20° C. to +80° C., changes ofsensitivity depending on temperature settled within 3.0% (within ±1.5%).As compared with the blank circle mark in FIG. 18 relating to the priorart, it is known that the temperature characteristic is extremelyimproved. Still more, since the magneto-optical devices in Table 1 areused in the optical magnetic field sensor probes employing the confocaloptical system shown in FIG. 1 to FIG. 5, the linearity of the outputwas evaluated, and an excellent linearity of 1.0% or less was shown upto about 500 Oe of the saturation magnetic field of the rare earth irongarnet material. Also, when applied in the optical magnetic field sensorprobe using bent fibers in FIG. 6 and FIG. 7 capable of receivingdiffracted light of high order, similarly, an excellent temperaturecharacteristic was exhibited.

The fundamental theory for obtaining such favorable results is asfollows. In the optical magnetic field sensor probe composed of anoptical system for receiving diffracted light of 0th order only by amagneto-optical device, the DC component VODC and AC component VOAC ofthe sensor output are expressed in the following equations. Herein, thevalues are standardized with the proportional constant defined as 1.

    V.sub.ODC =A+(1/2)CH.sub.IN.sup.2                          (Equation 1)

    V.sub.OAC =BH.sub.IN sin ωt+(1/2)CH.sub.IN.sup.2 sin (2ωt-π/2)                                        (Equation 2)

where constants A, B, C are

    A=cos.sup.2 θ.sub.F /2, B=cos θ.sub.F sin θ.sub.F)/Hs,C=sin.sup.2 θ.sub.F /2Hs.sup.2,   (Equation 3)

θ_(F) is the Faraday's rotational angle, and Hs is the saturationmagnetic field. The degree of modulation m₀ of the optical magneticfield sensor for receiving diffracted light of the 0th order only beingcomposed of rare earth iron garnet material to the fundamental wave isdefined by V_(OAC) /V_(ODC), and is hence determined from equation 1 andequation 2. The degree of modulation m₀ to the output fundamental wave wis expressed as follows as the function of temperature T:

    m.sub.0 (T)=B(T)H.sub.IN /(A(T)+C(T)H.sub.IN.sup.2 /2      (Equation 4)

On the other hand, in the optical magnetic field sensor composed of anoptical system for receiving diffracted light of all orders beingcomposed of a magneto-optical device, its output DC component V_(allDC)and AC component V_(allAC) are expressed as follows:

    V.sub.allDC =1/2                                           (Equation 5)

    V.sub.allAC =2BH.sub.IN,2B=sin 2θ.sub.F /Hs          (Equation 6)

Therefore, the degree of modulation m_(all) of the optical magneticfield sensor for receiving light of all orders is expressed as followsas the function of temperature T:

    m.sub.all (T)=4B(T)H.sub.IN                                (Equation 7)

From equation 4 and equation 7, once the composition of the garnetmaterial being used as the magneto-optical device is determined, theconstants A, B, C and their temperature changes are determined, andhence it is evident that the temperature dependence of the sensitivityof the optical magnetic field sensor is determined automatically.Therefore, as in the embodiments shown in Table 1, mainly by changingthe substitution concentration of element Gd, it is possible todetermine the composition of the rare earth iron garnet material with asmall temperature dependent change of sensitivity, in the opticalmagnetic field sensor probe for receiving diffracted light of highorder.

Moreover, from equation 7, receiving light in all orders, it is knownthat the temperature dependence of sensitivity does not depend on theapplied magnetic field. In other words, in the conventional sensoroptical system for receiving light of the 0th order only, if thecomposition of the rare earth iron garnet material is determined forminimizing the temperature dependent change of sensitivity according toequation 4 at a certain magnetic field intensity, since the square termof the magnetic field is included, if the applied magnetic field ischanged, the temperature dependence of the sensitivity is also changed.Therefore, by receiving diffracted light of up to a high order, it isconsidered possible to realize an optical magnetic field sensor notvarying in linearity due to temperature.

Accordingly, when employing a sensor optical system for receivingdiffracted light of high orders, in order to enhance the measuringprecision of the optical magnetic field sensor probe using rare earthiron garnet material as the magneto-optical device, it is necessary toimprove the composition of the rare earth iron garnet material in thesame manner as in the embodiment. The elements for replacing the rareearth elements are not limited to the combination of Bi+Gd+La+Y as inembodiment 7 in Table 1, but may include other substitution combinationsfor which it is easy to control the temperature characteristic disclosedso far. For example, combinations of rare earth elements Bi+Gd+Y, Bi+Gd,Bi+Tb, Bi+Yb+Tb, Bi+Y+Tb, Bi+Eu+Ho, Bi+Nd+Tb, Bi+Ho+Tb, Bi+Er+Pb, etc.,and further element Fe combined with elements replacing at least oneselected from the group consisting of Ga, Al, Sc, In, and Pt, can beused with such rare earth iron garnet material having such combinations,it is possible to compose the optical magnetic field sensor probe orsensors using bent optical fibers, decreased in temperaturecharacteristic of sensitivity and excellent in linearity, in the samemanner as in the invention.

In embodiment 7, meanwhile, La is used as the element for replacing forthe purpose of lattice matching, but it is also possible to use one ormore rare earth elements in the element R in formula 1. At this time, anon-magnetic material having no effect on the saturation magnetizationof the rare earth iron garnet material brings about favorable results.Alternatively, if other than a Ca--Mg--Zr substituted Gd₃ Ga₅ O₁₂crystal substrate differing in lattice constant is used as the garnetcrystal substrate, for lattice matching, replacing the element R informula 1 with one or more rare earth elements allows a rare earth irongarnet material of excellent linearity and temperature characteristic tobe grown.

Incidentally, this improvement of linearity and temperaturecharacteristic was recognized not only with a light source in the 0.8 μmband, but also at other wavelengths in the 1.3 μm band or the 1.5 μmband for passing through rare earth iron garnet material. Further, notonly at a frequency of 60 Hz, but also from a DC magnetic field tohundreds of MHz, the magnetic field could be measured at high precision.These were the results produced by growing the magneto-optical device onthe Ca--Mg--Zr substituted Gd₃ Ga₅ O₁₂ crystal substrate (SGGG crystalsubstrate), but similar results were obtained by varying the growthconditions on a Nd₃ Ga₅ O₁₂ substrate. It was also possible to grow thematerial epitaxially on these substrates by the vapor phase growthmethod.

In any growth method, depending on the growth conditions, the rare earthiron garnet material expressed in formula 1, and having the crystalcomposition with the value of x defined in a range of 0.84≦x≦1.10, thevalue of y in a range of 0.73≦y≦1.22, the value of z in a range of0.02≦z≦0.03, and the value of w in a range of 0.27≦w≦0.32, may be formedas a polycrystal without epitaxial growth, and such a polycrystalmagneto-optical device can be sufficiently used although the lightabsorption loss is sightly larger.

(Embodiment 8)

An eighth embodiment of the invention is described below by referring tothe accompanying drawings. It is a feature of this embodiment that themagnetic wall coercive force is 0.2 Oe or less. FIG. 13 shows therelation of the film thickness dependence to the magnetic wall coerciveforce (Hw), concerning the linearity of the optical magnetic fieldsensor probe in the weak magnetic field of the eighth embodiment of theinvention. That is, the linearity in the weak magnetic field of theoptical magnetic field sensor probe capable of receiving diffractedlight from the 0th order to a high order as shown in FIG. 1 or FIG. 2 isshown to depend greatly on the film thickness (d) of the magneto-opticaldevice to be used. As the film thickness of the magneto-optical deviceincreases, it is known that the linearity error in the weak magneticfield is extremely improved.

The fundamental reasons for obtaining such favorable results aredescribed below. The probable causes of linearity error in the sensoroutput are (1) the effect of receiving orders of diffracted light, (2)the non-linearity of the magnetization curve of rare earth iron garnetmaterial, and (3) the large magnetic wall coercive force of rare earthiron garnet material, and at the high magnetic field side, mainly (1)the effect of the light receiving condition of the diffracted light isconsidered to be significant.

Accordingly, improvement of linearity in the low magnetic field regionis specifically described below from the viewpoint of linearity error.In a low magnetic field region of 20 Oe or less where a negativelinearity error is large, the frequency of the sensor output wasanalyzed. As a result, in the low magnetic field, both the fundamentalwave (ω) and the third harmonic wave (3ω) were observed. The frequencyspectra of the sensor output observed in applied magnetic fields of 3.4Oe and 105 Oe are shown in FIGS. 14(a) and 14(b). From FIG. 14(a), 3ωbecame the same level as the noise shown in FIG. 14(b) as the appliedmagnetic field increased, and substantially there was no effect on thelinearity of the sensor output. Generally, when harmonic waves of anodd-number order of fundamental waves are generated, it is consideredthat a hysteresis loop is present at the input and output side. Thecause of this third harmonic wave is mainly (3), i.e., the largemagnetic wall coercive force of BiRIG crystal, among the three factorsmentioned above. When Hw is large, the magnetic wall hardly moves in thevery weak magnetic field of less than Hw, and the sensor output does notchange linearly. That is, the smaller the value of Hw, the betterconsidered the linearity of the sensor output in the low magnetic field.Therefore, to decrease the linearity error in the low magnetic fieldregion, it is known to decrease the Hw of the rare earth iron garnetmaterial.

Hence, fabricating optical magnetic field sensor probes by using rareearth iron garnet materials of different film thicknesses, the linearitywas measured. The results are shown in FIG. 13. Herein, plotted pointsare measured values, and the broken line and solid line are linearityerror curves calculated by using the value of Hw determined from themeasured results. The value of Hw (peak value) was determined byextrapolating the linear portion of the graph plotting the sensor outputvalues in the applied magnetic field, to zero of the sensor outputvalue, and the value of Hw =0.2 Oe was obtained at 40 μm, and Hw =0.03Oe at 60 μm. The linearity error curve was calculated on the basis ofthe following equation 8, in consideration of the term of Hw in equation7 of the degree of modulation m_(all) of the magnetic field sensor forreceiving light in all orders.

    m.sub.all =0,Ho<Hw

    m.sub.all =4B×(Ho-Hw),Ho≧Hw                   (Equation 8)

It is known from this embodiment that the negative linearity error isdecreased notably in the low magnetic field at the film thickness of 60μm. By using the rare earth iron garnet material of the thicker filmthickness of 80 μm, too, the linearity error in the low magnetic fieldwas decreased. On the other hand, the calculated value determined byconsidering Hw coincided very well with the measured value, and it isknown that the linearity error curve depends greatly on the value of Hw.

To measure at a precision of 1.0% or less in a magnetic field of 20 Oeor less, FIG. 13 tells it that is enough to use the magneto-opticaldevice having a value of Hw is 0.2 Oe or less from the theoreticalformula in equation 8 and experiment. Moreover, the smaller the value ofHw, it is confirmed, the more decreased is the linearity error in thelow magnetic field. The magneto-optical device used in FIG. 13 is a rareearth iron garnet material expressed in formula 1, but this filmthickness dependence of the magnetic wall coercive force is consideredto be applicable to all magneto-optical devices having magnetic domains.

(Embodiment 9)

A ninth embodiment of the invention is described below while referringto the accompanying drawings. A feature of the ninth embodiment lies inthe heat treatment of the magneto-optical device, and it makes use ofthe effect of the heat treatment on the linearity in a low magneticfield.

That is, by proper heat treatment of the rare earth iron garnet materialexpressed in formula 1, it is known that the linearity error in the lowmagnetic field region is decreased. The heat treatment is conducted inan electric furnace, by placing the rare earth iron garnet materialexpressed in formula 1, as shown in FIG. 15, on a ceramic tray 23 bycontacting with the growth surface of the rare earth iron garnetmaterial 21 on the SGGG crystal 22 used in the substrate 20. Unless thegrowth surface is covered in this manner, heat treatment fails, and thefilm surface of the rare earth iron garnet material will be damaged andbroken. The heat treatment temperature was 900° C., 950° C., 1000° C.,1050° C., and 1100° C., and at each temperature, the heat treatment timewas 5 hours, 10 hours, 15 hours, 20 hours, and 25 hours, in order toevaluate the dependence on time. The surface state of the rare earthiron garnet material after heat treatment was observed by using anoptical microscope. In the heat treatment time of 10 hours, and at theheat treatment temperature of 900° C. to 1000° C., there was noparticular change in the crystal surface, but at 1100° C., the filmsurface was damaged at several positions. At 1200° C., it was known thatthe film was broken.

Using the heat treated rare earth iron garnet material, the opticalmagnetic field sensor probe shown in FIG. 2 was fabricated, and thelinearity was measured. Results are shown in Table 2 and Table 3. Table2 shows the results of measurement of sensor output linearity error andmagnetic wall coercive force by varying the heat treatment temperature,and Table 3 shows the results of measurement of linearity error andmagnetic wall coercive force by varying the heat treatment time. In thetables, the linearity error denotes the low magnetic fieldcharacteristic in the magnetic field range of 20 Oe or less, and the xmark shows failure in measurement due to damage of the film.

                  TABLE 2                                                         ______________________________________                                                900° C.                                                                      950° C.                                                                        1000° C.                                                                        1050° C.                                                                      1100° C.                         ______________________________________                                        Linearity error                                                                         ≦1.5                                                                           ≦1.2                                                                           ≦1.0                                                                          ≦0.5                                                                          x                                     (%)                                                                           Magnetic wall                                                                           0.35    0.3     0.2    0.05   x                                     coercive                                                                      force (Oe)                                                                    ______________________________________                                         (Annealing Time: 20 hours)                                               

                  TABLE 3                                                         ______________________________________                                                     5     10     15      20   25                                                  hours hours  hours   hours                                                                              hours                                  ______________________________________                                        Linearity error (%)                                                                          ≦1.5                                                                           ≦1.2                                                                          ≦1.0                                                                         ≦0.5                                                                        x                                    Magnetic wall coercive                                                                       0.3     0.2    0.1   0.05                                      force (Oe)                                                                    ______________________________________                                         (Annealing temperature: 1050° C.)                                 

It is known that the linearity error at 20 Oe or less is decreased asthe heat treatment temperature is higher (Table 2) and the heattreatment time is longer (Table 3). However, if heated for more than 25hours at a higher temperature than 1100° C., the film of the rare earthiron garnet material expressed in formula 1 is damaged and broken, andan optical magnetic field sensor probe cannot be composed. Therefore,from the observation of the surface state after heat treatment and theabove results, to realize a linearity error of 1.0% or less, it isappropriate to heat the material at 1000° C. to 1050° C. for 15 hours ormore, and control the magnetic wall coercive force to be under 0.2 Oe.

As clear from the description herein, according to the magneto-opticaldevice and sensor optical system of the invention, excellent linearityand a superior temperature characteristic are realized over a widermagnetic field range than in the prior art, and an optical magneticfield sensor probe of small size and high reliability capable ofmeasuring magnetic fields at high precision can be presented.

What is claimed is:
 1. An optical magnetic field sensor probe fordetecting a magnetic field, comprising:an input optical fiber forinputting light into the optical magnetic field sensor probe, a firstdrum lens, a polarizer, a magneto-optical device, an analyzer, a seconddrum lens, and an output optical fiber for emitting output light fromthe optical magnetic field sensor probe, the input optical fiber, thefirst drum lens, the polarizer, the magneto-optical device, theanalyzer, the second drum lens and the output optical fiber beingarranged in order along a light path such thatthe input optical fiber,the first drum lens, the magneto-optical device, the second drum lensand the output optical fiber together form a confocal optical system,and the analyzer has transmission and polarization directions that aredifferent than the polarizer.
 2. An optical magnetic field sensor probeas recited in claim 1, further including a lens holder for holding thefirst drum lens and the second drum lens.
 3. An optical magnetic fieldsensor probe as recited in claim 1, wherein the first drum lens and thesecond drum lens each have a single refractive index of 1.517 or more.4. An optical magnetic field sensor probe of claim 2, wherein said firstdrum lens and said second drum lens are lenses having a refractive indexof 1.517 or more.
 5. An optical magnetic field sensor probe as recitedin claim 2, wherein the first drum lens and the second drum lens eachhave a single refractive index of 1.517 or more.